Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of wellbore testing and sample taking instruments. More particularly, the invention relates to designs for such instruments which reduce a possibility of the instrument and/or conveyance device becoming stuck in a wellbore.
2. Background Art
When drilling a wellbore through earth formations for the purpose of producing hydrocarbons, frequently the wellbore operator requires information concerning formation and wellbore parameters, such as fluid pressure and fluid content of the various formations penetrated by the wellbore. Such pressure and fluid content information is used for, among other purposes, determining a depth at which to set casing, determining which formations are likely to be commercially productive of hydrocarbons or whether to set casing at all.
Various instruments are known in the art for taking formation fluid pressure measurements and/or formation fluid samples. Many of these instruments are designed to be conveyed at one end of an armored electrical cable (xe2x80x9cwirelinexe2x80x9d conveyed). Other types of instruments may be conveyed by coiled tubing, drill pipe or similar conveyances. These instruments typically include an elongated instrument housing adapted to traverse the wellbore. The instrument housing includes therein a probe adapted to be extended from the housing and placed in externally sealed engagement with the wall of the wellbore at the position of a formation to be tested. Various flowlines, pressure transducers and sample chambers are disposed in the instrument housing and are adapted to cause fluid to be withdrawn from the selected formation while pressure and fluid composition properties are measured. In some cases a sample of the formation fluid will be directed to a storage tank for ultimate removal from the wellbore and subsequent analysis at the earth""s surface. Examples of such formation pressure measuring and sample testing instruments are described in U.S. Pat. No. 6,058,773 issued to Zimmerman et al. and U.S. Pat. No. 4,936,139 issued to Zimmerman et al.
One particular concern associated with substantially all formation pressure measuring and sampling instruments such as the ones described in the above references is that the instrument must be stopped in the wellbore in order to take a sample and/or make a pressure measurement. Stopping the instrument in the wellbore substantially increases the risk of the instrument and/or means of conveyance becoming stuck in the wellbore. Mechanisms for becoming stuck include debris settling out of the drilling fluid and lodging between the instrument and the wellbore wall, differential pressure between the drilling fluid in the wellbore and the formation being tested, and the conveyance becoming xe2x80x9ckeyseatedxe2x80x9d in the wall of the wellbore. So called xe2x80x9ctractorxe2x80x9d devices have been developed to prevent wellbore tools from sticking in the wellbore. Examples of such tractor devices include U.S. Pat. No. 5,954,131 issued to Sallwasser on Sep. 21, 1999 and U.S. Pat. No. 6,179,055 issued to Sallwasser et al. on Jan. 30, 2001, the entire contents of both are hereby incorporated by reference. These tractor devices convey a tool along a wellbore using a cam system to lock against the borehole wall.
What is needed is a device for enabling continued motion of the conveyance and a substantial portion of the instrument while the tool conducts wellbore operations, such as deploying a probe to make a formation pressure measurement and/or fluid test. One such device is described for example in U.S. Pat. No. 4,600,059 issued to Eggleston et al. The device disclosed in this reference includes a telescoping section coupled between a wireline conveyed fluid testing instrument and the armored electrical cable. When the testing instrument is deployed to test a particular earth formation, and is thus stationary, the armored electrical cable may be kept in continuous motion by repeated extension and retraction of the telescoping section. This is known in the art as xe2x80x9cyo-yoingxe2x80x9d the cable. Yo-yoing the cable requires the cable operator to pay very close attention to a winch control system to avoid too much upward and/or downward motion of the cable for operating the telescoping section. It is desirable to have a telescoping section for a wellbore test instrument which does not require cable yo-yoing.
It is desirable to have a wellbore instrument, such as a formation fluid pressure and/or sampling instrument, which enables substantially continuous motion of a well logging conveyance in order to prevent sticking and reduce the duration of wellbore operations. This combination of a wellbore instrument in a continuous motion enables economically combining wellbore options, such as a combined pressure/fluid sample test instrument with other types of well logging instruments that make measurements while moving along the wellbore. Typically, such xe2x80x9cmoving measurementsxe2x80x9d have not been combined with formation pressure and sampling instruments to operate simultaneously because the former are adapted to make measurements while moving along the wellbore, and the latter, as previously explained, must be stopped. Examples of the former include, without limitation, acoustic devices, resistivity devices and nuclear porosity and lithology measuring devices.
One aspect of the invention is a well logging instrument which includes a lower housing having therein a formation testing system adapted to be operated in an axially fixed position in a wellbore. The instrument also includes an upper housing adapted to be operatively coupled to a well logging conveyance. The instrument includes an axial extension mechanism operatively coupled between the lower housing and the upper housing. The extension mechanism is adapted to controllably extend and retract to lengthen and shorten the instrument, respectively.
A method for testing an earth formation according to another aspect of the invention includes moving a logging instrument axially along a wellbore by operating a logging conveyance coupled to an upper end of the instrument. A testing system adapted to test the earth formation at a fixed axial position along the wellbore is deployed, while continuing to move the conveyance along the wellbore. A length of the logging instrument between the conveyance and the testing system is increased by operating an axial extension mechanism disposed between the conveyance and the testing system, while continuing to move the conveyance along the wellbore. The earth formation is tested, the testing system is retracted; and the axial extension mechanism is then retracted. In one embodiment, tension between the instrument and the conveyance is measured, and the axial extension mechanism is extended at a rate adapted to maintain the tension substantially constant.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.